The present invention relates to a patch antenna, and more particularly, to a patch antenna suitable for an antenna mounted on a transportable electronic device and an antenna mounted on a vehicle such as an automobile.
Currently, various antennas are mounted on a vehicle such as an automobile. Such an antenna, for example, an antenna for GPS (Global Positioning System) or an antenna for SDARS (satellite digital audio radio service) is used.
The GPS is a satellite positioning system using artificial satellites. In the GPS system, electric waves (GPS signal) are received from four artificial satellites (hereinafter, referred to as “GPS satellite”) among twenty four GPS satellites orbiting around the earth, a positional relation and a time error between a mobile object and the GPS satellites are measured on the basis of the received electric waves, and a position or an altitude of the mobile object on a map is high precisely calculated on the basis of triangulation.
Recently, the GPS is used for a car navigation system detecting a position of a traveling automobile and comes into wide use. The car navigation device includes a GPS antenna for receiving a GPS signal, a processor for detecting a present position of a vehicle by processing the GPS signal received by the GPS antenna, a display for displaying the position detected by the processor on a map, and the like. A planar antenna such as a patch antenna is used as the GPS antenna.
The SDARS (Satellite Digital Audio Radio Service) is a digital broadcasting service using a satellite (hereinafter, referred to as “SDARS satellite”) in the United States. That is, in the United States, a digital radio receiver receiving a satellite wave or a terrestrial wave from the SDARS satellite to provide digital radio broadcasting has been developed and put in practical use. Currently, in the United States, two broadcasting stations of XM and Sirius have provided radio programs more than total 250 channels throughout the whole country. The digital radio receiver is generally mounted in a mobile object such as an automobile, receives the electric wave in the frequency band of about 2.3 GHz, and provides the radio broadcasting. That is, the digital radio receiver is a radio receiver capable of providing the mobile broadcasting. Since the frequency of the reception electric wave is about 2.3 GHz, the reception wavelength (resonance wavelength) λ at that time is about 128.3 mm. The terrestrial wave is formed in the manner that the satellite wave is received by an earth station, the frequency of the received satellite wave is slightly shifted, and the wave is re-sent in a linearly-polarized wave. That is, the satellite wave is a circular-polarized wave, but the terrestrial wave is the linearly-polarized wave. In addition, the planar antenna such as the patch antenna is used as the SDARS antenna in the same manner as the GPS antenna.
The antenna device for XM satellite radio receives the circular-polarized electric wave from two geostationary satellites, and receives the electric wave by using terrestrial linear-polarized equipments in a blind zone. On the other hand, the antenna device for Sirius satellite radio receives the circular-polarized electric wave from three orbiting satellites (synchro type), and receives the electric wave by the use of the terrestrial linear-polarized equipments in the blind zone.
Since the electric wave in the frequency band of about 2.3 GHz is used in such digital radio broadcasting, an antenna device receiving the electric wave is required to be installed outdoors. As a digital radio receiver, there are a receiver built in a vehicle, a receiver mounted on a roof, and a transportable receiver which has a battery as a power supply.
The transportable digital radio receiver is built in a transportable electronic device such as a transportable acoustic device. In addition to a digital tuner for receiving the digital radio broadcasting, for example, a optical disk drive for reproducing an optical disk such as a compact disk (CD), a lamp, a speaker are integrally built in a case of the transportable digital device. In the transportable electronic device, it was proposed that a patch antenna is built in an openable-closable door-type cover for inserting or removing the optical disk (e.g., see Japanese Patent Publication No. 2005-110198A).
The related-art first patch antenna 10 will be described with reference to FIGS. 1 to 3. FIG. 1 is a perspective view of the related-art patch antenna 10. FIG. 2(A) is a plane view of the patch antenna 10, FIG. 2(B) is a front view of the patch antenna 10, FIG. 2(C) is a left side view of the patch antenna 10, and FIG. 2(D) is a bottom view of the patch antenna 10. FIG. 3 is a cross-sectional view taken along Line III-III in FIG. 2(A).
The patch antenna 10 includes a dielectric substrate 12 having a substantially rectangular parallelepiped shape, an antenna radiation electrode (radiation element) 14, a ground electrode (ground conductor) 16, and a rod-shaped feeding pin 18.
For example, ceramic materials with high permittivity made of barium titanate are used for the dielectric substrate 12. The dielectric substrate 12 has a top surface (front surface) 12u, a bottom surface (back surface) 12d, side surfaces 12s. In the example in the figures, the corners of the side surfaces 12s of the dielectric substrate 12 are chamfered. In the dielectric substrate 12, a substrate hole 12a penetrating from the top surface 12u to the bottom surface 12d is formed in a position where a feeding point 15 is disposed.
The antenna radiation electrode (radiation element) 14 is formed of a conductive film and formed on the top surface 12u of the dielectric substrate 12. The antenna radiation electrode (radiation element) 14 has a substantially square shape. The antenna radiation electrode (radiation element) 14 is formed, for example, by a silver-pattern printing method.
The ground electrode (ground conductor) 16 is formed of a conductive film and formed on the bottom surface 12d of the dielectric substrate 12. The ground electrode (ground conductor) 16 has a ground hole 16a being substantially concentric with the substrate hole 12a and having a larger diameter than that of the substrate hole 12a. 
The feeding point 15 is provided at a position shifted in an x-direction and a y-direction from the center of the antenna radiation electrode 14. The feeding point 15 is connected to one end 18a of the feeding pin 18. The feeding pin 18 is extended downwardly through the substrate hole 12a and the ground hole 16a at a distance from the ground electrode (ground conductor) 16.
In the related-art first patch antenna 10, solder is used as the feeding point 15. For this reason, the feeding point 15 has a convex shape protruding upwardly from the main surface of the radiation electrode 14. That is, as shown in FIG. 3, the patch antenna 10 has a height H1 from the ground electrode 16 to the feeding point 15.
As shown in FIG. 4, as a feeding pin, there was also proposed the related-art second patch antenna 10A in which a rivet pin 18A having a head portion 181 provided at one end 18a thereof and a rod-shaped body portion 182 extending from the one end 18a thereof to the other end 18b thereof are used. In the related-art second patch antenna 10A, the head portion 181 of the rivet pin 18A is connected to the antenna radiation electrode 14 by soldering while the head portion 181 of the rivet pin 18A protrudes on the main surface of the antenna radiation electrode 14. For this reason, the connected part becomes the feeding point 15 and has a convex shape. That is, the patch antenna 10A shown in FIG. 4 has a height H2 from the ground electrode 16 to the feeding point 15. The height H2 is larger than the height H1 (H2>H1).
In addition, as shown in FIG. 5, in the related-art third patch antenna 10B, a circular antenna radiation electrode (radiation element) 14A is used instead of the square antenna radiation electrode (radiation element) 14.
A surface-mounting patch antenna in which the feeding pin does not pass through the mounting substrate and devices can be mounted on the surface of the mounting substrate was proposed (e.g., see Japanese Patent Publication No. 2005-260875A). In the surface-mounting patch antenna disclosed in Patent Document 2, a concave portion being substantially concentric with a substrate hole and having an inner diameter lager than the diameter of the substrate hole is formed on the back surface of a dielectric substrate. The other end of a feeding pin is formed in the substantially same plane with an exposure surface of a ground electrode provided on the back surface of the dielectric substrate.
As disclosed in the Japanese Patent Publication No. 2005-110198A, when the patch antenna is provided in the cover, it is required that the space for mounting the patch antenna is reduced as small as possible. Accordingly, it is very effective to reduce the height of the patch antenna.
However, in the related-art patch antennas 10, 10A, and 10B inclusive of the patch antenna disclosed in the Japanese Patent Publication No. 2005-260875A, since the feeding points 15 protrude relatively high from the main surfaces of the antenna radiation electrodes (radiation element) 14 and 14A in the convex shape, it is restricted to reduce the heights H1 and H2 of the patch antenna 10, 10A, and 10B.